Systems for printing printed wiring boards with liquid photopolymers are known, for example, U.S. Pat. No. 4,436,806, Issued Mar. 13, 1984 to F. J. Rendulic, et al. However, such prior art systems have several basic shortcomings, which are overcome by the present invention. For example, stencil printing systems cannot produce high resolutions such as necessary in modern printed circuit arts with very fine, closely spaced conductive wires. In photoimaging arts, off contact printing has many disadvantages, including high energy and corresponding heat and slowness problems because of the oxygen inibition of polymers. Furthermore for high resolution it is necessary to use collimated radiation, with corresponding large size and optical complexities.
Dry film processes require expensive polymers and auxiliary cover sheets, and many times need heat treatment or special adherence techniques, particularly if applied onto rough surfaces, where there is a tendency to entrap air bubbles that deteriorate adherence and resolution. Neither are they tolerant to coating thickness variations, which may be critical in many applications.
Liquid polymers are not heretofore well adapted to a wide variety of coating thicknesses. Generally, thin coatings are desired to reduce radiation energy for photoimaging, and to increase resolution, and to save polymer cost. Thus, commercial polymer lines with high quality control are not generally available to produce thick coatings, which provide unsatisfactory photo response in the presence of fillers, pigments and other ingredients, even if they were adapted to attain physically the desired thicknesses. My co-pending application, Ser. No. 650,109, Filed Sept. 13, 1984 thus provides a preferred liquid photopolymer for attainment of thick film layers, as are necessary if printed circuit boards are to be processed having thick conductive traces such as 0.004 in. (0.01 cm) extending from the insulation of the substrate. Because the photopolymer printing art has been generally confined to uncritical very thin film layers, generally stencilled onto the workpiece, the photoimaging techniques for high resolution and precise registration of optical images have not been highly developed, nor the ability to deal with thick layers, nor the attainment of good high resolution solder mask coatings consistent with precision printed wiring boards.
Special problems are posed by printed wiring boards with through holes, where it has not been feasible to use liquid photopolymers that run off into holes and prevent tenting over the holes with solder mask layers, for example.
A whole line of problems are introduced in processing apparatus because of inherent high costs of former systems and the limitations on processing speeds and product variations. Thus satisfactory photopatterning systems for high speed, low cost production of products including complex two sided printed wiring boards with semi-skilled operators have not heretofore been available in the art.
It is therefore an object of this invention to overcome the foregoing inefficiencies and problems of the prior art and to produce an improved high speed, low cost photoimaging system using liquid photopolymers, yet capable of high precision, high resolution production of a variety of product lines with semi-skilled operators. Other objects, features and advantages of the invention will be found throughout the following description, drawings and claims.